Deafening snowflakes

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While most of us consider falling snowflakes to make hardly a sound new acoustic research reveals that just below the water surface they're heard as an earsplitting racket by marine animals and sonar equipment.

A team of researchers from four American universities, including Johns Hopkins, published these findings, in the Journal of the Acoustical Society of America, after analysing recordings they made underwater during winter storms. The culprits are thought to be oscillating bubbles, too small and short-lived to be seen by the naked eye.

According to the report, as a snowflake falls onto a body of water, it deposits a tiny amount of air just beneath the surface. Before the bubble reaches the surface and pops, it sends out a piercing sound.

"If you submerge a pocket of air trapped in a snowflake, that pocket of air cannot just sit there," explained Andrea Prosperetti, a co-author of the article. "We know that snowflakes are made of many ice crystals arranged together in such a way as to leave a lot of space for air because the density of a snowflake is about one-tenth the density of water. That means nine-tenths of the volume of a snowflake is air. When a snowflake strikes the water, it melts, and the air inside is freed up as a bubble. When that bubble oscillates, it makes noise."

This screeching sound, ranging from 50 to 200 kilohertz, is too high-pitched to be heard by human ears, which generally pick up nothing higher than 20 kilohertz. But the snowflake noise could be quite annoying to porpoises and other aquatic animals that can detect the higher frequencies, said Lawrence A. Crum, from University of Washington's Applied Physics Laboratory in Seattle and lead author of the study. Falling snow can add 30 decibels to underwater noise levels, Crum said.